Recuperation of valuable metals from photographic baths



March 15, 1938. H. G. DOFFIN 2,110,930

REGUPERATION OF VALUABLE METALS FROM PHOTOGRAPHIC BATHS Filed Dec. 17, 1955 2 Sheets-Sheet 1 Fig-Eb T'I'I'IlV/Zi-d I fl TTMEMEY March 15, 1938. H, DOFF|N 2,110,93Q

RECUPERATION OF VALUABLE METALS FROM PHOTOGRAPHIC BATHS Filed Dec. 17, 1955 2 Sheets-Sheet 2 Figl "q v nuen'or:

6'. pol /in Patented Mar. 15,1938 I PATENT OFFICE RECUPERATION F YALUABLE METALS FBOMPHOTOGRAPHIC BATES ilenri Georges Doflln, Chalet, France Application December 11, 1935, Serial No. 54,906 In France December 20, 1934 2 Claims. (01. 204-16) It is known that fixing photographic baths are made of a solution of sodium hyposulflte, to which there is generally added an acidifyin agent (sodium bisulflte or acetic acid) and often other 5 bodies, such as potassium or chromium alum. As

a rule, they are thrown out or subjected to a recuperation treatment for recovering silver, as soon as the proportion of silver reaches about from 3 to 6 grammes per liter.

10 It has already been suggested to electrolyze worn fixing baths, with a view to extracting silver therefrom. But in the known methods, it has always been endeavoured to obtain a rapid deposition of the silver that is to be recovered, by

working with high densities of current, while violently stirring the electrolyte. When the percentage of silver in the bath drops below a certain value, these high densities of current produce a blackening of the silver deposit and the transformation of apart of the silver into sul-' flde which-remains in suspension in the bath,

whereby the latter must be filtered in the course For this purpose, according to the present in- 35 vention, the tension between electrodes is maintained below the critical value for which a deposit of silver would become black in an electrolysis bath containing less than 0.1 gr. of silver per liter.

40 Said critical tension depends upon the composition of the bath that is treated. In practice, it ranges between 0.3 and 0.5 volt, according to the case. In all cases, for a bath of a given composition, the electrolytic tension must be kept 45 very constant, with a variation of at most 0.02 or 0.03 volt, below the critical maximum value. As a matter of fact, it is not advantageous to operate under a tension substantially lower than the critical maximum tension, since the rate of 50 deposition of' silver decreases rapidly when the tension between the electrodes is reduced, even very little, below the critical tension. When the silver deposit keeps a creamy white shade, in a bath containing less than 0.1 gr. of silver per 55 liter, the electrolytic tension is surely sufllcient- Iy below the critical maximum tension, without however being too low. When the tension is too low the silver deposit is of a chalky white. The tension that gives good deposits in baths containing very low percentages of silver is suitable 5 for baths containing. a higher percentage of silver.

The low value of the tension between the electrodes is compensated for by correspondingly increasing the area of the electrodes. When, for m a given bath, the tension between the electrodes has been adjusted to a suitable value, electrolysis can be pursued without supervising it, with a density of current-which varies between 15 and 0.1 milli-ampere per square decimeter for pro- 15 portions of silver ranging between 3- and 0.1 gramme per liter. The intensity of the electrolysis current drops as the percentage of silver in the bath drops, which permits of knowing the residuary proportion. 20

The electrolytic treatment may be performed either in the fixing vessel or tank, or in a special tank. It may be performed by means of a battery consisting of two metals in contact and immersed in the bath itself. These metals are chosen in such manner that the couple has .a uniform electromotive force, but little lower than the critical maximum electrolysis tension, and that the working of the battery does not give birth .to salts capable of interfering with the photographic fixing. Such a battery may for instance be constituted by copper and inoxidable steel. I may also make use of an external source of current capable of maintaining a tension which is uniform with variations of at most 0.02 or 0.03 volt,'between the electrodes, whatever be the variations of intensity of the electrolytic current. I may for instance make use of a transformer having an adjustable primary, combined with a rectifier, or again a battery having an 40 electromotive force corresponding to the low tensions of electrolysis to be maintained.

Preferred embodiments of the present invention will be hereinafter described, with reference to the accompanying drawings, given merely by way of example, and in which:

Fig. 1 is a diagram of a device permitting. to obtain, from alternating current, a continuous or substantially continuous current at the desired tension. I Fig. 2 shows another current supply device, including a battery;

Fig. 3 shows the arrangement of'a cathode;

Fig. 4 shows the arrangement of an anode;

' composed of a row of carbon rods.

Fig. 5 is a perspective view of the whole electrolytic tank; I

Figs. 6 and 7 show simplified arrangements, respectively.

The device shown in Fig. 1 shows a transformer I connected to an alternating current distribution system. The primary ill of this transformer has ten input elements, connected to an equal number of contacts carried by a switch 2, respectively. The secondary ll ofthis transformer is itself connected to two dry rectifiers 3, of the "metal-oxidized metal type. In the output circuit of the rectifiers are mounted, in series, a milliammeter 4 and, in shunt, a millivoltmeter 5. The terminals of this circuit are connected respectively with the anode and the cathode of the electrolytic tank. The rectifiers work on a permanent shunt 50, which avoids any rise of the tension when the baths are practically exhausted from silver, the current accordingly becoming zero in the tank. The operation of switch 2 permits of varying, by amounts of hundredth of a volt, the continuous tension that can be utilized.

The arrangement of Fig. 2 includes a battery, including a carbon 6, adapted to be depolarized by air, and an iron tube 1, surrounded by an insulating sleeve 8 open only at the bottom. The electrolyte is, for instance, a solution of sodium chloride of 10% strength. In a closed circuit, there is formed a very bulky iron hydrate, which drops to the bottom of vessel 9, which, for this purpose is made of great height. The electromotive force of this battery approximates 0.45, a value which is quite appropriate for carrying out the electrolysis according to the invention. Such a battery does not seem to be capable of any other application. A millivoltmeter 90 and a small potentiometer 9| permit of adjusting accurately the tension at the terminals of the electrolytic tank.

This tank may consist of the fixing vessel itself. For instance, in the case of a vertical tank for films, I make use of an anode 12, extending over the whole width of the tank (Fig. 4) It is These rods are fixed at the top in a head made of cast antimonious lead IZI.

The cathode i3 (Fig. 3) consists of a thin sheet of pickled inoxidizable steel, from which silver is removed by scraping, the deposit being brittle and being not welded to steel. I may also make use of a cathode made of a composition of matter which is not alterable under the conditions of working and which is sufiiciently brittle in order that it may be broken, with the amount of silver it carries. As an average, a square decimeter of electrode maintains one liter of solution below 0.5 decigramme of silver per liter.

The whole of the electrodes has a thickness which is but some centimeters. It is supported on one side of the tank, the cathode being kept at a distance of some centimeters from the wall,

by a tube ll of inoxidizable steel, which is placed like a film rod and to which are welded two rods ii of the same metal, provided with hooks at their respective ends-and immersed vertically in the liquid. The cathodic blade i3, which is provided with two holes is carried by these two rods, below the liquid level. The anode is also fixed to tube l4, through two inoxidizable rings, fitted internally with rubber, for electric insulation. The insulated lead wire which connects the anode to the positive terminal of the source penetrates into tube i4 through a hole and issues on the side of the tank, close to the negative conductor.

In order to permit an easy initial adjustment of the electrolysis tension, there is provided a small test cathode 18, consisting of a small blade of Inoxidizable steel, fixed to tube I4 through an elastic conductor I I6. By removing the test cathode from the bath. it is possible to examine the colour of the deposit. If it is of a creamy white, the tension is within the proper limits. Any blackening shows that thetension is too high; any whitening shows that the tension is too low.

An analogous device may be provided in horizontal dishes, with a carbon anode on one side and a cathode of Inoxidizable steel resting upon the bottom through small pieces of rubber. But it is easier to cause the bath to fiow through a vertical tank or a separate tank.

This separate tank consists of a rectangular vessel of glass, provided with electrodes extending through the whole of its volume. to wit: an anode, consisting of two rows of rods i2 of carbon, disposed laterally in opposite relation, and assembled so as to form a whole by a rectangular frame I 2|, of lead, and, in the interval, a plurality of small blades forming cathode elements, maintained at a distance of about. 5 centimeters from one another. These small blades are each suspended through two holes to the curved teeth of two racks l3l, of inoxidizable steel, the level at which they are supported being below the liquid level. I have found that the best possible volume for the tank is from 8 to liters, but of course tanks of greater volume might be employed. The tank is provided at its bottom with a discharge tube, to which can be fitted a rubber tube I32, which is lifted and fixed to the edge of the tank. when it is desired to prevent the discharge of the bath from the tank. The electrolyzing apparatus may be given other forms, so as to conform to particular cases.

The consumption of current-is very low. Theoretically, it is 0.25 kilowatt for 1 kilogramme of silver precipitated and about 1000 liters of bath treated. In actual practice, it may be somewhat higher due to the consumption of the transformer under no load. v

The simplified arrangement of Fig. 6 consists of a voltaic element, constituted by two metals in contact and immersed in the bath itself. This couple consists for instance of a blade of inoxidizable steel i1 and a small block of copper i8, tightly held in a bent and perforated end of blade IS, with a felt pad I9 interposed. The whole rests upon the bottom of the tank, with the interposi tion of rubber pieces 20.

The modification of Fig. 7 can be used in vertical tanks. Inoxidizable steel is employed in the form of a blade, the perforated base of which is curved arounda small block of copper 22, with the interposition of a porous mass 23. A welded nickel wire connects the copper part with the steel part. The whole is supported in such manner as to dip in the liquid of the tank.

The plant is completed by accessories: a salt weighing densimeter, adapted to control the percentage of hyposulfite in the bath, and a colorimeter device for measuring the acidity which must correspond to a good fixing. This device includes two drop-bottles one of which contains a solution of a colouring matter corresponding to the nature of the bath: methyl red for potassium alum baths, purple bromocresol for the others, the other bottle containing the acidifying solution. The plant further includes a test-tube of a section of 1 square centimeter. The colouring matter is concentrated enough for masking the colour the present invention as comprehended within of the bath itself. ,A drop of colouring matter is dropped in the test tube filled with the bath. 11' this bath is too much basic; drops of the aciditying body are added until the desired coloration is obtained (the pH is 4.8 for potassium alum and 6 for chromium alum). The test tube and the drop-bottles are calculated in such manner that, to each drop, there corresponds 5 grammes per liter of acidifying body to be added to the bath.

While I have, in the above description,- disclosed what I deem to be practical and efllcient embodiments of the present invention, it should be well understood that I do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the scope of the appended claims.

What I claim is:

1. A process for the electrolytic recovery of silver contained in photographic fixing baths, which comprises maintaining the tension between the electrodes at avalue slightly below the critical value for which a silver deposit assmnes a brownish'colouring in an electrolysis bath containing less than 0.1 gramme of .silver per liter, this critical tension ranging between 0.3 and 0.5 volt.

2. A process according to claim 1 which further comprises maintaining a very uniform electrolysis tension, with a variation ranging between 0.02 15 and 0.03 volt below the critical tension.

l HENRI GEORGES DOFEIN. 

